1. Field of the Invention
The present invention relates to phosphors used in cathode ray tubes (CRTs) for liquid crystal displays, and, more particularly, to a new family of phosphors having the yttrium aluminum garnet crystal structure.
2. Description of Related Art
Phosphors used for addressing the amorphous-silicon in the Hughes liquid crystal light valve (LCLV) must meet the following conditions:
Spectral output as closely matched to the amorphous silicon (.alpha.-Si) response as possible;
Maximum radiant energy output in the spectral range of interest;
Decay time less than approximately 10 milliseconds measured from 100% to 10% levels;
Average particle size suitable for high resolution operation - less than about 6 .mu.m; and
High resistance to either thermal or life-induced degradation.
The spectral responsivity of .alpha.-silicon used in the LCLV peaks at about 740 nm. Consequently, for optimum sensitivity, the incoming light which is used to activate the .alpha.-silicon photosensitive layer should provide as close a match to the responsivity curve as possible.
Many phosphors have been investigated in an attempt to find a phosphor having the correct characteristics in regard to spectral emission, decay time, efficiency, and small particle size. Such phosphors include aluminum oxide:Cr, cadmium sulfide:Ag, zinc cadmium sulfide:Ag, zinc phosphate:Mn, yttrium oxysulfide:Eu, yttrium aluminum oxide:Eu, etc.
One existing far-red phosphor (by "far-red" is meant about 600 to 800 nm) used in luminescent lamps demonstrated a very close match to the .alpha.-silicon responsivity. It consists of a lithium aluminum oxide activated with iron (LiAlO.sub.2 :Fe). In the commercially available material, the iron concentration is about 0.5%. However, this phosphor, when used in CRTs, has a much longer decay time than can be tolerated, approximately 30 milliseconds.
Yttrium aluminum garnet (YAG) phosphors are well-known in the art. They have various activators such as terbium, cerium, europium, etc. In YAG:Eu, the energy is distributed between the 591, 608, 630, and 710 nm lines, with lesser peaks at about 649 and 692 nm. Because of this spectral output, this phosphor does not efficiently turn on the light valve.
No commercially available phosphors were found to satisfy the .alpha.-silicon LCLV requirements.